The present invention relates to a method and a system for supplying pressurized gas from a liquefied-gas (LG) storage tank.
LG systems are widely used in residential, agricultural, and industrial settings and they are expected to be a reliable source of energy, to operate safely, continuously, and to constantly supply guaranteed output. One critical performance criterion of LG systems is the delivery of a constant, stable and reliable flow of vaporized gas to the burners.
Some commonly used systems and methods of vaporization employ over-capacity storage tanks and vaporizers. The over-capacity storage tanks are expensive and provide inconsistent vapor pressure, especially during extreme ambient conditions. They also waste space, call for surplus gas stock and unnecessarily large and expensive storage area.
In one known system, the heat of vaporization is supplied by convection with respect to the ambient heat. However, this requires large heat-convection surfaces, according to the demand for the vaporized gas. Moreover, such systems are incapable of delivering gas at pressures exceeding that of the tank pressure.
In another known system, an external vaporizer is used to heat and vaporize the liquefied gas, with the vaporized gas being recirculated to the storage tank. The vaporized gas supplied to the consumer is delivered via a separate line connected to the storage tank, according to consumer demand. The recirculation of vaporized gas requires a large installation, and correspondingly high investment cost and maintenance expenses. Alternatively, the vaporized gas can be delivered directly to the consumer. However, various mechanical and control-related failures, as well as reduced consumer demand, can cause liquid-phase liquefied gas to be introduced to or to be condensed in the consumer delivery line. Liquid-phase liquefied gas in the vaporized gas is a major problem for many consumers. Consequently, such a process scheme generally requires the addition of vapor/liquid separation equipment for providing the consumer with solely vaporized gas. This equipment increases system complexity, size and cost, and introduces additional reliability and safety problems.
In my pending U.S. patent application Ser. No. 09/674,700, which is hereby incorporated for all purposes as if fully set forth herein, a system and method are disclosed in which the liquefied gas is circulated through an external heat exchanger and returned to the storage tank as a heated liquid. The sensible heat of the heated liquefied gas provides all of the requisite heat for vaporizing the gas (or at least a substantial portion thereof), which occurs within the storage tank. The vaporized gas is delivered via a separate vapor line connected to the storage tank, according to consumer demand.
The above-described system has numerous advantages with respect to other known systems. However, the storage tank operates under equilibrium or near-equilibrium conditions, such that the vapor produced is substantially saturated vapor. Consequently, any cooling that occurs in the supply line results in condensation of a portion of the vaporized gas. The system is not particularly suitable for high-pressure applications, because the entire system, including the storage tank and lines, must be pressurized according to the demand pressure. This means that the entire system operates under high pressure, a safety issue that preferably should be avoided.
There is therefore a recognized need for, and it would be highly advantageous to have a method and a system for supplying vaporized gas on consumer demand by producing superheated gas, thereby guaranteeing a reliable flow in any weather conditions and allowing for an improved fire safety than in systems known heretofore. It would be of further advantage if such a system would be simple and energy-efficient.
The present invention is a method and a system for supplying pressurized gas from a liquefied-gas (LG) storage tank. According to one aspect of the invention, the method includes the steps of: (a) providing a system including: (i) a storage tank for storing liquefied gas, the tank having a lower liquid region and a vapor region thereover; (ii) a heat exchanger external to the storage tank; (iii) pumping means driven by a liquid; and (iv) a line for directly transferring the vaporized gas from the heat exchanger to the consumer; (b) pumping at least a portion of the liquefied gas from the lower region to the heat exchanger using the pumping means; (c) heating the liquefied gas in the heat exchanger to produce the vaporized gas; and (d) supplying the vaporized gas directly to the consumer, according to consumer demand.
According to another aspect of the present invention there is provided a system for supplying vaporized gas to a consumer, including: (a) a storage tank for storing liquefied gas, the tank having a lower liquid region and a vapor region thereover; (b) a heat exchanger external to and fluidly communicating with the storage tank, within which heat exchanger, a heat exchange liquid and the liquefied gas come into indirect heat-exchange relation; (c) pumping means, driven by a liquid, for pumping at least a portion of the liquefied gas from the storage tank through the heat exchanger; (d) a gas supply line, operatively connected to the heat exchanger, for directly transferring the vaporized gas from the heat exchanger to the consumer upon demand; and (e) a control system associated with the heat exchanger and configured to control a rate of heat supply to the liquefied gas so as to vaporize the liquefied gas.
According to further features in the described preferred embodiments, the vaporized gas is a superheated gas.
According to still further features in the described preferred embodiments, the vaporized gas produced in step (c) is supplied directly to the consumer.
According to still further features in the described preferred embodiments, the liquid for driving the pumping means is used for heating the liquefied gas in step (c).
According to still further features in the described preferred embodiments, the liquid is heated externally to the system of step (a).
According to still further features in the described preferred embodiments, the method further includes heating the line to prevent condensation of the vaporized gas.
According to still further features in the described preferred embodiments, the heat for heating the line is provided by the liquid from the pumping means.
According to still further features in the described preferred embodiments, the liquid includes water.
According to still further features in the described preferred embodiments, the liquid consists of water.
According to still further features in the described preferred embodiments, the method further includes the step of (e) controlling a flow of the liquid so as to reduce an amount of liquid-phase gas in the line.
According to still further features in the described preferred embodiments, the controlling includes reversing the flow of the heat-exchange liquid.
According to still further features in the described preferred embodiments, the controlling includes increasing a temperature of the heat-exchange liquid.
According to still further features in the described preferred embodiments, the control system is configured to control a rate of heat supply to the liquefied gas so as to superheat the liquefied gas.
According to still further features in the described preferred embodiments, the system is designed and configured such that the heat exchange liquid is used as the liquid for driving the pumping means.
According to still further features in the described preferred embodiments, the system further includes: (f) heating means for heating the gas supply line so as to prevent condensation of the vaporized gas.
According to still further features in the described preferred embodiments, the heating means includes a line of the liquid for driving the pumping means, and the gas supply line and the heat-exchange liquid line are configured in heat-exchange relation.
According to still further features in the described preferred embodiments, the pumping means includes a hydraulic motor, connected to the pumping means, and driven by the liquid.
According to still further features in the described preferred embodiments, the pumping means includes a pump selected from the group consisting of a vane pump, an impeller pump, and a gear pump.
According to still further features in the described preferred embodiments, the heat exchanger and the pumping means are solely mechanical, such that any electrical components are disposed remotely from the storage tank.
According to still further features in the described preferred embodiments, the line of the liquid for driving the pumping means is disposed in side-by-side heat-exchange relation with the gas supply line.
According to still further features in the described preferred embodiments, the line of the liquid for driving the pumping means is disposed in coaxial heat-exchange relation with the gas supply line.
According to still further features in the described preferred embodiments, the system further includes (f) an auxiliary heat exchanger, operatively connected to the gas supply line, and disposed in series and downstream with respect to the heat exchanger, for ensuring full vaporization of the liquefied gas.
According to still further features in the described preferred embodiments, the auxiliary heat exchanger is heated by the heat exchange liquid.
According to still further features in the described preferred embodiments, the control system is further designed and configured to change a flow direction of the heat exchange liquid.
According to still further features in the described preferred embodiments, the control system is further designed and configured to change a flow direction of the heat exchange liquid in response to an indication from at least one indicator, such that fully vaporized gas is delivered to the consumer.
According to still further features in the described preferred embodiments, the control system is designed and configured to increase a temperature of the heat exchange liquid in response to an indication from at least one indicator, such that fully vaporized gas is delivered to the consumer.